Magnetic stimulation has been used extensively for researching and treating a large number of clinical conditions. Magnetic stimulation can be implemented by passing a time varying current through a coil. As known to those skilled in the art, the time varying current creates a time varying magnetic field in the area near the coil. By placing the coil near or in contact with a patient, the time varying magnetic field passes through at least a portion of the patient. The time varying magnetic field induces an electrical field which in turn causes an electrical current (or eddy current) within the patient. The eddy current interacts with and is capable of stimulating the patient's neural tissue. For example, if the electrical field has a large negative gradient of sufficient duration, it can cause nerve fibers in the patient to depolarize and initiate an action potential. If the electrical field has a large positive gradient, it can cause nerve fibers to hyperpolarize, and may even be able to block action potential propagation.
Magnetic stimulation is similar to electrical stimulation in that both use a current to stimulate a patient's neural tissue. However, magnetic stimulation has several advantages over electrical stimulation. Magnetic stimulation is relatively painless and can easily be applied through an electrically insulated portion of the patient, such as a skull. Further, magnetic stimulation does not require an invasive procedure, and if the magnetic stimulator is powerful enough, magnetic stimulation may not even require contact with the patient.
A major limitation of magnetic stimulation is the ability to design a coil that is capable of efficiently inducing a desired electric field or other constraint at a specified stimulation location within a patient or other individual. It is desirable that the induced electric field be controlled, for example, such that stimulation takes place at the stimulation location and does not interfere with or excite any neighboring locations within the patient.
Many researchers have conducted optimization studies in an attempt to optimize coil designs to improve the ability to control the stimulus location for magnetic stimulation of both the central and peripheral nervous systems. These optimization studies have examined maximizing the electric field or the electric field gradient for various tissues by altering coil shapes, sizes, orientations, number of coils used, etc. A major problem of coil designs generated based on these optimization studies is that they assume a general (round or elliptical) shape of the optimal coil and then alter a limited number of parameters affecting the assumed shape in an attempt to achieve a stimulation goal. Because the general shape is assumed and not determined, there is no guarantee that the coil design found is globally optimal. In addition, traditional magnetic stimulation coils have been designed without considering the effects of heterogeneity of the conductivity of surrounding tissues. Further, traditional magnetic stimulation coils are limited in their ability to effectively control the magnetic stimulation at more than one location.
Thus, there is a need for an optimization technique which can be used to determine an optimal coil shape for a magnetic stimulation device such that desired objectives can be realized. Further, there is a need for an optimization technique to determine an optimal coil solution in which the general coil shape is not assumed. Further, there is a need for an optimization technique for determining a coil which considers the heterogeneity of the conductivity of tissues surrounding a desired tissue location. Further, there is a need for an optimization technique to determine an optimal coil shape such that a desired electric field profile can be simultaneously provided to a plurality of tissue locations.